Pcie Standard Specification -

The PCIe specification is defined by its (measured in GigaTransfers per second, or GT/s) and its encoding method , which determines how much raw data becomes usable bandwidth. Heqing Zhu - Data Plane Development Kit (DPDK) | PDF

Example: A PCIe x4 card fits into any x4, x8, or x16 slot. But a PCIe x16 card does not fit into an x4 slot (the plastic barrier prevents it). pcie standard specification

If you have ever plugged in a graphics card, an NVMe SSD, or a high-speed network adapter, you have used PCI Express (PCIe). But beneath the familiar slot shapes lies a complex, evolving standard specification that dictates the speed, power, and future of nearly every high-performance computer. The PCIe specification is defined by its (measured